The levitation class was about to begin. Children gathered behind James Percy and Benjamin Stratton at the Avenue of the Stars to fly their broomsticks. But instead of yelling "Wingardium leviosa" to raise the brooms, they said "One, two, three, jump!"

OK, so Percy and Stratton can't really cast spells, but they do make magic happen - on stage. They're the actors in Potted Potter, a drama which tells the entire Harry Potter saga in 70 minutes.

Potted Potter, which was here two years ago, is called the "unauthorised Harry experience" because ultimately it's a classic comedy duo. This is why the show will be funny whether or not the audience know the story.

"As long as you know Harry Potter's a boy wizard, you're fine," says Percy, who plays the sensible Harry. Stratton is the silly one who does all the other characters wrong, on purpose. "I mean, she's a beautiful thing, my Hermione, but she's no Emma Watson," he says. "I'm playing it how I would just imagine this person being, so it's deliberately done in a funny way."

In fact, the story's inaccuracies and even the onstage accidents are the core of the show. Stratton says the first time he performed, he was so flustered he forgot to take off Ron's wig. "So he played Voldemort with ginger hair," laughs Percy.

Stratton says: "Things go wrong, but we're allowed to let the audience in on it. It's not awkward because we make it part of the show." And those are the bits we look forward to the most, because that's when we really get to have fun.

Unlike a regular play, the pair don't stick to a fixed script. They make things up and interact with the audience. "It makes each show seem like opening night," says Percy. The audience also plays a crucial role, the "third actor", he says. "The more you put into it as an audience, the more you'll get out of it. If you find something funny, let us know. We want to hear it loud and proud." He says the best audiences are those who are the loudest.

When the pair first rehearsed, they tried to make each other laugh by trying different voices and characters. They believed if they could make each other laugh, they could make the audience laugh. "All I basically do when I go on stage is try to annoy him as much as I can," says Stratton - although Percy says Stratton is annoying all the time, not just on stage.

But laughter has its price. The pair say high physical and vocal stamina are needed to keep up with the energy of the show. "You've got to really get a good night's sleep and drink a lot of coffee," says Percy, holding up an empty Starbucks cup. "We're not very rock 'n' roll at all. We should be going out and doing parties and crazy things ... but we don't. We just go back and sleep a lot."